1. FIELD OF THE INVENTION
This invention relates to a method of fabrication and to the resulting structure for a birefringent electromagnetic transmission line (such as a single-mode optical fiber) that is adapted to maintain the state of polarized radiation propagating through said transmission line.
2. PRIOR ART
As will be known to those skilled in the art, certain electromagnetic transmission means permit the propagation therethrough of two different modes of polarized radiation having orthogonal orientations and slightly different phase velocities relative to one another. When the transmission means is subjected to a physical disturbance or force (e.g. temperature or pressure), energy (i.e. power) may be electromagnetically coupled from one polarization mode to another. Examples of the foregoing phenomenon may generally be found by referring to the following U.S. Pat. Nos.: 3,439,974, Apr. 22, 1969, 4,111,050, Sept. 5, 1978.
Because of the difference of phase velocities (with regard to the directions of orientation), the relative interference patterns of the propagations at the terminal point of the transmission means may appear different from one another. Depending upon the magnitude and location of the aforementioned physical disturbance, a superposition of the interference patterns may produce respective fringes which are undesirably shifted and reduced in contrast. What is more, should the state of the transmitted polarization be intended to provide an indication of the physical disturbance, then the resulting inter-mode coupling may consequently cause a distortion of the information represented thereby.
By way of example, apparatus which are known to employ transmission means comprising a length of optical fiber to transmit information signals in the form of polarized radiation include gyroscopes, hydrophones, various communication systems, automotive sensors, and the like. It may be desirable that the fiber optic transmission means be characteristically insensitive to various external and environmental physical disturbances or forces, so as to avoid the electromagnetic coupling of energy from one polarization mode to the other and to prevent an upset in the state of the polarized radiation propagating therethrough. A discussion of a single-mode, fiber optic transmission line that is capable of maintaining the state of polarized radiation being transmitted therethrough can be found in the document entitled "Fiber-Ring Interferometer: Polarization Analysis", by Ulrich and Johnson, Vol. 4, No. 5, Optics Letters (Optical Society of America) May, 1979. An additional discussion which teaches an optical fiber transmission line that is particularly fabricated so as to be relatively insensitive to the applications of external forces, whereby to preserve the state of polarized radiation traversing the fiber, may be found by referring to patent application Ser. No. 46,968 filed June 8, 1979, but now abandoned and entitled "Method And Apparatus For Producing An Optical Fiber In Which The Polarization Changes Of Polarized Light Therein Are Insensitive To Externally Applied Forces".
Documents which describe the formation of an optical (e.g. planar) waveguide by radiation processing techniques are those entitled "Optical Waveguides Formed By Proton Irradiation of Fused Silica", by E. Ronald Schineller et al, Vol. 58, J. Opt. Soc. Am., pp. 1171-1176, Sept., 1968 and "Properties of Ion Bombarded Fused Quartz for Integrated Optics" by R. D. Standley et al., Vol. 11, Applied Optics, pp. 1313-1316, June, 1972.